This research utilizes the tunable nature of synchrotron radiation x-rays from the high energy electron-positron storage ring SPEAR to investigate the nature of binding of Ca 2 ions in biological systems, specifically the membrane bound protein assembly constituting the acetyl choline receptor by the use of anomalous diffraction. From heavy metal atoms (lanthanides) substituted for Ca 2 ion, further information will be obtained concerning the molecular arrangment of the receptor in relation to the lipid bilayer. It is proposed to label agonist molecules with an absorbing atom so that the structural details of the agonist-receptor atom interaction may be further elucidated. Theoretical studies of the statistical mechanics of protein-lipid interactions are also being carried out.